Plasma arc cutting with swirl flow



' Jan. 30, 1968 R. J. WICKHAM' ETAL' 3,366,772

PLASMA ARC CUTTING WITH SWIRL FLOW Filed July 20, 1964 2 Sheets-Sheet 1 POWE R SUPPLY WORK INVENTORJ' ROBERT J. WICKHAM WILLIAM P. K

ATTORNEY Jan.

J. WICKHAM ETAL PLASMA ARC cu'rwmdwna ISWIRL FLOW Filed July 20, 1964' 2 s'heets-shee'i'z i! X v v 8:: g K ii o 9 w (J 8E. 4 w

- r- Lj Y 9, 3 l I O v V 1' k I o i I 3 1 I 8 )I m I I/ I I, l 8 i N I I I I g a: o w o 05 o g Y v+ I I I T I Q'I S d NI HUHSSBUd dll 300813313 INVENTOR5 vBV ' ROBERT J. WICKHAM WILLIAM F. KEANE Y United States Patent Oh ice 3,356,772 Patented Jan. 30, 1968 3,366,772 PLASMA ARC CUTTING WITH SWERL FLQW Robert .I. Wiclrham, Fairlield, and William P. Keane,

ArlingtomNJL, assignors to Union Carbide Qorporation, a corporation of New York Filed July 20, 1964, Ser. No. 383,788 5 Claims. (Cl. 219-121) This invention relates to a plasma arc process and, more particularly, to plasma are cutting of metals.

Plasma arcs of the type referred to in this invention are distinguishable from ordinary electric arcs in that such plasma arcs are characterized by a plasma stream having a high energy density relative to an ordinary electric arc. Such high energy density plasma stream is created by passing a gas between a non-consumable electrode and a cooled nozzle having a central arc constricting passage. The gas is heated by the energy of an are established from the non-consumable electrode to create the plasma which usually contains neutral gas, ions, and electrons at high energy levels. In plasma are cutting, the plasma is directed to a workpiece through the constricting passage of the nozzle at a high temperature and momentum, which combine to cut the work.

A typical plasma arc is described in Are Torch and Process, U.S. Patent No. 2,806,124.

Various modifications have been made in the plasma are cutting process in an effort to improve cut quality, speed of cutting, and economics of the process, among other things. Investigators have studied the effects of the way the gas is introduced into the arc zone in hopes of discovering a way to improve cut quality and speed. These investigators have introduced the gas in a swirling or vortical manner in an effort to make some improvement in the basic process. However, up until now none of the investigators, in the field, have fully realized the phenomenon created by swirl flow of the arc gas nor have they fully recognized how to control such phenomenon to improve the plasma are cutting process.

It is a primary object of the invention to provide a plasma arc process wherein gas is introduced into the arc zone in a particular swirling mode.

It is another object to provide a plasma arc cutting process wherein gas is introduced into the arc zone in a particular swirling mode to achieve optimum cutting conditions.

A further object is to provide such a plasma are cutting process wherein gas is introduced into the arc zone in a particular swirling mode and then passed out of such zone in a multiplicity of streams.

Another object is to provide a plasma arc process wherein arc gas is introduced in such a manner so as to produce a swirling .gas vortex having a pressure of between zero gage and zero absolute at the center thereof when there is no arc present.

These and other objects will either be pointed out or become apparent from the following description and drawings wherein:

FIGURE 1 is a schematic diagram of a typical system for practicing the invent-ion; and

FIGURE 2 is a curve of pressure at the electrode tip vs. gas flow obtained by practicing the invention.

While for purposes of simplification, the description will be directed to the application of the inventive concept to plasma arc cut-ting, such concept should not be construed as being limited thereto. For example, the plasma are produced by the invention may also be used for heating gases such as inert gas, air, oxygen, etc.

Typical apparatus for carrying out the invention is shown schematically in FIGURE 1. Referring to such figure, a torch noted generally at T is connected to one side of a power supply P. The torch, shown diagrammatically, includes a nonconsumable electrode 1 consisting of a holder 2 and an insert 4, inside of a nozzle 5. The insert material can be, for example, zirconium or tungsten. The non-consumable electrode 1 is in axial alignment with the center passage 7 in the nozzle and set back from the mouth of such passage so as to form an arc chamber 9 therein. The center passage 7 is surrounded by a plurality of passages 11. This type nozzle will be referred to throughout this disclosure as a multiport nozzle and is the preferred type nozzle for use with the process of the invention. However, a single port nozzle, that is a nozzle without the surrounding passages 11, may also be used in the practice of the present invention.

In plasma are cutting, a workpiece W is connected to the other side of the power supply P. However, if the plasma are produced by the present invention is to be used to heat gases such as argon, helium, air, oxygen, nitrogen, etc., the other side of the power supply P will be connected to the nozzle 5.

In operation, the arc gas is introduced into the torch T through tangentially arranged gas ports 13, positioned not more than 1 inch behind the arcing surface 15 of the electrode 1. If the gas is introduced beyond 1 inch from the arcing surface 15, the pressure condition necessary for practicing the invention will not be obtained. Preferably four symmetrically arranged gas ports are provided. The gas is introduced at at least sonic velocity and is caused to swirl around the elect-rode 1 in an annular stream defined by the annular chamber 16 liorm'ed between the electrode 1 and the wall 17 of the nozzle 5. The annular stream should be between in. and in. in width. It has been found that in order to achieve the improvement in cutting speed and cut quality not only should the gas be introduced not more than 1 inch behind the arcing surface but also the annular stream must be maintained within the limits indicated above.

We have found, in order to improve cutting speed and better cut quality, the pressure at the arcing point 15 of the electrode 1 or the center of the swirling gas must be between zero gage and zero absolute measured when there is no are. This condition is obtained according to our invention by introducing the gas tangentially at at least sonic velocity and maintaining the swirling annulus between 4; and -in. in diameter. These conditions are obtained without an are being present. When the arc is established, the pressure condition will change but, nevertheless, it has been discovered that the best arcing condition is achieved when the above conditions are established prior to initiation of the arc.

In the preferred embodiment the arc and are gas are caused to pass through a multiport nozzle. The are column passes through the center passage and the remainder of the arc gas passes through the peripheral holes. It unexpectedly has been found that swirl flow of the type described above, in combination with a multiport nozzle, forces a larger proportion of the gas through the peripheral holes than through the center hole. It is theorized that this is due to the pressure gradient created in the arc chamber. Higher presure is created over the peripheral holes than over the center, thus causing the unexpected gas distribution.

It was found that the flow rate of cold gas through the peripheral holes was about two times that through the central hole. The term cold gas is used to indicate that there was no are present. This occurs although the total peripheral hole area is only /3 the center hole area. When radial or axial gas introduction was used, it was found that flow in the center and peripheral holes divided according to their relative areas; this demonstrates that swirl flow gas introduction according to the invention coutributes to the desirable multiport action.

3 At typical are conditions, i.e. with the are on, with a /8 in. (4 x 12 M) diameter multiport nozzle cutting /2- inch carbon steel at 140 i.p.rn. gas distribution was as follows:

Total air flow c.f.h 250 Center hole flow c.f.h 5O 8 peripheral holes flow c.f.h 20f) Arc voltage volts 150 Are current amperes 275 The main difference between conditions obtained with a multiport nozzle and single port nozzle is the average temperature of the gas in the center hole.

The gas fiow in the center passage is much less when using a multiport nozzle with swirl flow and consequently the effluent gas attains nearly twice the gas temperature observed with a single port nozzle. This doubling of the average gas temperature is a remarkable advantage because, for example, oxygen at 7000 K. is not ionized whereas oxygen at 13,000" K. is far into the ionization region. The combination of swirl flow of the invention and multiport nozzles permits extremely high average gas temperatures in the center passage and at the same time permits larger volumes of gas at high velocity to sweep past the electrode and nozzle and thus inhibit secondary arcing between the electrode and nozzle.

While the swirl flow of the invention has remarkable utility with multiport nozzles, it also improves operation with single port nozzles. The high volume, high velocity flow of gas inhibits secondary arcing. The swirl of the invention, in combination with a single port nozzle, produces an arc column which remains in axial alignment with the electrode even when going around corners and turns. This is especially significant in shape cutting. For example, with a single port nozzle and swirl flow producing positive pressures at the electrode arcing surface, it was found that when a circle was cut in a workpiece the side of the cut would have beveled portions. With swirl flow producing pressures between zero gage and zero absolute, all portions of the sides of the cut were equally straight.

The family of curves in FIGURE 2 summarizes a number of experiments performed with multiport nozzles (designated by the M) and single port nozzles. The size designation for the nozzles shown in H6. 2 is interpreted as follows. The first figure is the diameter of the center passage given in thirty-seconds (32ds) of an inch, i.e. for a 4 x 12 M nozzle the center passage has a diameter of & or of an inch. The second figure represents the length of the central passage, i.e. or /8 of an inch in length. The M designates multiport which means 8 peripheral holes around the central passage. In each case, gas was tangentially introduced through four (4) symmetrically arranged orifices not more than 1 inch behind the electrode arcing surface at at least sonic velocity. The swirling gas was passed through an annular chamber having diameter of between Ms in. to about in.- usually and preferably about A in. The electrode set ba k from the central passage orifice was .219 in. Under these conditions it was found that for each nozzle, as the flow rate was increased, it was possible to approach zero absolute pressure at the arcing surface of the electrode and that when a cut was made under these conditions the highest speed and best quality was obtained at the minimum point of the curve for each nozzle. It was also found that good cuts and cutting speed could be obtained at lower gas fiows near the minimum point. For example, with a 4 X 12 M multiport nozzle, that is a nozzle having /s-in. diameter center passage and 8.026 diameter peripheral holes on a A -in. hole circle, best cuts were obtained at about 400 c.f.h. which produces 6 p.s.i.g. pre sure. However, satisfactory cuts could be obtained at 250 to 400 c.f.h.

The following example is provided to illustrate the improvements made by the present invention and so that 4 those skilled in the art will have a clear understanding of how to practice the invention.

One-half inch carbon steel was connected in circuit between a power supply and a torch provided with a 4- x 12 M multiport nozzle. The electrode was a non-consumable electrode comprising a zirconium insert in a water-cooled copper holder. The electrode was set back 0.219 in. from the mouth of the center passage in the nozzle. The diameter of the arc chamber between the electrode and walls of the nozzle was in. About 250 c.f.h. of air was introduced through four symmetrically placed tangential holes about 1 inch behind the arcing surface of the electrode. The pressure at the electrode tip was measured at about 5 p.s. 3. An arc was then established by high frequency between the electrode and work, at 275 amperes and 140 volts. The cutting speed was 140 i.p.m. Under identical conditions except that the annular chamber 16 was greater than /8 in. in diameter, the cutting speed was only i.p.m.

While the invention has been described with reference to a preferred embodiment, it is to be understood that certain modifications may be made in the practice of the invention without departing from the inventive concept disclosed and all modifications should be construed as being within the scope of the claims. For example while the multiport nozzles described herein all had eight peripheral holes and a center passage area three times the total area of the peripheral holes, it is possible to modify this relationship without affecting the practice of the invention.

What is claimed is:

1. A method of producing a plasma are between two electrodes at least one of which is a non-consumable electrode having an arcing portion thereon which comprises:

(a) tangentially introducing a gas at a selected flow rate and at at least sonic velocity behind the arcing portion of said non-consumable electrode and up to about 1 inch therebehind to provide a swirling annulus of gas;

(b) maintaining the width of said annulus of gas between about to about A; in.;

the combination of steps (a) and (b) producing a pressure of between zero gage and zero absolute at the arcing portion of said nonconsumable electrode when there is no arc;

(c) establishing an electric are between said two electrodes while maintaining the conditions in steps (a) and (b);

(d) introducing at least a portion of said swirling annulus of gas and said are into a passage the walls of which confine said are to produce a high energy density are column.

2. A method for cutting with a plasma are established between a non-consumable electrode and a workpiece, said non-consumable electrode having an arcing portion, which comprises:

(a) tangentially introducing a gas at a selected flow rate and at at least sonic velocity behind the arcing portion of said non-consumable electrode and up to about 1 inch therebehind to provide a swirling annulus of gas;

(b) maintaining the width of said annulus of gas between about to about A; in.;

the combination of steps (a) and (b) producing a pressure of between zero gage and zero absolute at the arcing portion of said non-consumable electrode when there is no arc;

(c) establishing an electric arc between said non-consumable electrode and said workpiece while maintaining the conditions in steps (a) and (b);

(d) introducing at least a portion of said swirling annulus of gas and said are into a passage the walls of which confine said are to produce a high energy density are column.

3. A method for cutting with a plasma are established between a non-consumable electrode and a workpiece, said non-consumable electrode having an arcing portion at least partially surrounded by a cooled multiport nozzle, said nozzle having a central passage and a plurality of peripheral holes surrounding said central passage, which comprises:

(a) tangentially introducing a gas at a selected flow rate and at at least sonic velocity behind the arcing portion of said non-consumable electrode and up to about 1 in. and behind to provide a swirling annulus of gas;

(b) maintaining the width of said annulus of gas between about 2 to inch;

the combination of steps (a) and (b) producing a pressure of between zero gage and zero absolute at the arcing portion of said non-consumable electrode when there is no arc;

(c) establishing an electric arc between said nonconsumable electrode and said workpiece while maintaining the conditions in steps (a) and (b);

(d) passing a portion of said swirling annulus of gas and said are through said central passage and the remainder of said swirling annulus of gas through said peripheral holes in said nozzle; and

(e) directing such arc and gas effluent from said nozzle against said work to be cut.

4. A method according to claim 3 wherein at least three times as much gas is passed through said peripheral passages as is passed through said central passage when said are is established.

5. A method according to claim 3 wherein said annulus of gas is about A in.

References Cited UNITED STATES PATENTS 2,770,708 11/1956 Briggs. 2,768,279 10/ 1956 Rava. 2,806,124 9/1957 Gage. 2,922,869 1/ 1960 Giannini et a1. 2,960,594 11/1960 Thorpe. 3,027,446 3/ 1962 Browning. 3,027,447 3/ 1962 Browning et a1. 219- 3,082,314 3/1963 Arata et al. 219-75 3,304,719 2/1967 Ducati 2l9-75v RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner. 

3. A METHOD FOR CUTTING WITH A PLASMA ARC ESTABLISHED BETWEEN A NON-CONSUMABLE ELECTRODE AND A WORKPIECE, SAID NON-CONSUMABLE ELECTRODE HAVING AN ARCING PORTION AT LEAST PARTIALLY SURROUNDED BY A COOLED MULTIPORT NOZZLE, SAID NOZZLE HAVING A CENTRAL PASSAGE AND A PLURALITY OF PERIPHERAL HOLES SURROUNDING SAID CENTRAL PASSAGE, WHICH COMPRISES: (A) TENGENTIALLY INTRODUCING A GAS AT A SELECTED FLOW RATE AND AT LEAST SONIC VELOCITY BEHIND THE ARCING PORTION OF SAID NON-CONSUMABLE ELECTRODE AND UP TO ABOUT 1 IN. AND BEHIND TO PROVIDE A SWIRLING ANNULUS OF GAS; (B) MAINTAINING TUBE WIDTH OF SAID ANNULUS OF GAS BETWEEN ABOUT 1/32 TO 1/8 INCH; THE COMBINATION OF STEPS (A) AND (B) PRODUCING A PRESSURE OF BETWEEN ZERO GAGE AND ZERO ABSOLUTE AT THE ARCING PORTION OF SAID NON-CONSUMABLE ELECTRODE WHEN THERE IS NO ARC; 